This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A120375 #56 Sep 08 2022 08:45:25
%S A120375 4,6,16,24,30,54,96,178,274,1332,2766,3060,4204,17736,190062,223536,
%T A120375 260400,683080
%N A120375 Integers k such that 2*5^k - 1 is prime.
%C A120375 See comments for A057472. Examined in base 12, all n must be even and all primes must be 1-primes. For example, 1249 is 881 in base 12.
%C A120375 a(16) > 2*10^5. - _Robert Price_, Mar 14 2015
%F A120375 a(n) = 2*A002958(n).
%e A120375 a(1) = 4 since 2*5^4 - 1 = 1249 is the first prime.
%p A120375 for w to 1 do for k from 1 to 2000 do n:=2*5^k-1; if isprime(n) then printf("%d, %d ",k,n) fi od od;
%t A120375 Select[Range[0, 100], PrimeQ[2*5^# - 1] &] (* _Robert Price_, Mar 14 2015 *)
%o A120375 (PARI) isok(k) = ispseudoprime(2*5^k-1); \\ _Altug Alkan_, Sep 22 2018
%o A120375 (Magma) [n: n in [0..2800] |IsPrime(2*5^n - 1)]; // _Vincenzo Librandi_, Sep 23 2018
%Y A120375 Integers k such that 2*b^k - 1 is prime: A090748 (b=2), A003307 (b=3), this sequence (b=5), A057472 (b=6), A002959 (b=7), A002957 (b=10), A120378 (b=11).
%Y A120375 Primes of the form 2*b^k - 1: A000668 (b=2), A079363 (b=3), A120376 (b=5), A158795 (b=7), A055558 (b=10), A120377 (b=11).
%Y A120375 Cf. also A000043, A002958.
%K A120375 nonn,more
%O A120375 1,1
%A A120375 _Walter Kehowski_, Jun 28 2006
%E A120375 More terms from _Ryan Propper_, Mar 28 2007
%E A120375 a(14) from Herman Jamke (hermanjamke(AT)fastmail.fm), May 02 2007
%E A120375 a(15) from _Robert Price_, Mar 14 2015
%E A120375 a(16)-a(18) from _Jorge Coveiro_ and _Tyler NeSmith_, Jun 14 2020